1) Field of the Invention
The present invention relates to an optical transmitter particularly suitable for wavelength-division multiplexing (WDM) transmission systems.
2) Description of the Related Art
With an increase in the use of the Internet, WDM transmission systems using WDM transmitters have been improved. The WDM transmission systems are being recently requested to have the function of setting transmission lines in wavelength units. Because of this, the application of wavelength-selective switches to the WDM transmitter is being studied so that line settings can be readily switched in wavelength units.
FIGS. 8A and 8B both illustrate the function of a wavelength-selective switch. The wavelength-selective switch 100A shown in FIG. 8A is a device capable of outputting the wavelengths of a wavelength-division multiplexed signal through its arbitrary ports. That is, the wavelength-selective switch 100A includes a single input port 101 and nine output ports 111 to 119, and if a wavelength-division multiplexed signal is input through the input port 101, the switch 100A can output arbitrary wavelengths through arbitrary ports of the output ports 111 to 119. For example, if a signal with multiplexed wavelengths λ 1 to λ 5 is input through the input port 101, the wavelengths λ 2, λ 1, λ 5, λ 3, and λ 4 can be output through the output ports 111, 113, 114, 116, and 118.
In addition, as shown in FIG. 8B, a wavelength-selective switch 100B may be constructed so that it becomes the reverse of the wavelength-selective switch 100A shown in FIG. 8A. In this case, the wavelength-selective switch 100B includes nine input ports 121 to 129 and a single output port 131, and if arbitrary wavelengths λ 1 to λ 5 are input through arbitrary ports of the input ports 121 to 129, a wavelength-division multiplexed signal can be output through the output port 131.
In addition to the function of switching ports, the wavelength-selective switch has the function of variably attenuating light intensity. Therefore, the wavelength-selective switch is able to compensate for a difference in light intensity between wavelengths by using the variable attenuator function. Note that examples of wavelength-selective switches are disclosed in patent document 1 (U.S. Pat. No. 6,549,699) and non-patent documents 1 (Jui-che Tsai et al., “A Large Port-Count 1×32 Wavelength-Selectable Switch Using a Large Scan-Angle, High Fill-Factor, Two-Axis Analog Micromirror Array” OECC 2004, Tul.5.2) and 2 (D. M. Marom et al., “Wavelength-selectable 4×1 switch with high spectral efficiency, 10 dB dynamic equalization range and internal blocking capability” OECC 2003, We4.P.130).
FIGS. 9 to 11 illustrate WDM transmitters 200, 210, and 220 employing wavelength-selective switches such as those shown in FIGS. 8A and 8B, respectively. These WDM transmitters 200, 210, and 220 are dynamic optical add-drop multiplexers (DOADMs) constructed in a way that arbitrary wavelengths can be input or output through their arbitrary ports. Similar dynamic optical add-drop multiplexers are disclosed in non-patent document 3 (D. M. Marom et al., “64 Channel 4×4 Wavelength-Selectable Cross-Connect for 40 Gb/s Channel Rates with 10 Tb/s Throughput Capacity” OECC 2003).
The WDM transmitter 200 shown in FIG. 9 is constructed so that the wavelength-selective switches 201, 202 are disposed opposite each other. The first wavelength-selective switch 201 is a 1×N wavelength-selective switch (where N≧2), which consists a single input port and N output ports. The second wavelength-selective switch 202 is an N×1 wavelength-selective switch (where N≧2), which consists N input ports and a single output port.
One of the N output ports of the first wavelength-selective switch 201 is connected to one of the N input ports of the second wavelength-selective switch 202. The remaining output ports of the first wavelength-selective switch 201 are used as drop ports, whereas the remaining input ports of the second wavelength-selective switch 202 are used as add ports.
In this arrangement, among the wavelengths of a wavelength-division multiplexed signal input to the input port of the first wavelength-selective switch 201, wavelengths that should be passed through the WDM transmitter 200 are output from the output port of the second wavelength-selective switch 202 through a transmission line leading to the second wavelength-selective switch 202. On the other hand, wavelengths to be branched are output through arbitrarily selected drop ports, and wavelengths to be added are input through the add ports of the second wavelength-selective switch 202 and are output from the output port of the second wavelength-selective switch 202.
The monitor 203 is used to monitor the optical power of each of the wavelengths of the optical signal output from the second wavelength-selective switch 202. The control circuit 204 equalizes the optical powers of the wavelengths of the optical signal output from the second wavelength-selective switch 202, by controlling the variable attenuator function of each of the wavelength-selective switches 201 and 202, based on the optical powers of the wavelengths monitored by the monitor 203.
The WDM transmitter 210 shown in FIG. 10 includes an optical combiner 211, a first wavelength-selective switch 212, a second wavelength-selective switch 213, a monitor 214, and a control circuit 215. The optical combiner 211 branches an input signal light into two and outputs them to the input ports of the wavelength-selective switches 212, 213, respectively.
The first wavelength-selective switch 212 consists of a single input port and N output ports. One of the two signals branched in the optical combiner 211 is input to the input port of the first wavelength-selective switch 212, and wavelengths to be dropped are output through the output ports arbitrarily selected as drop ports.
The second wavelength-selective switch 213 consists of N input ports and a single output port. The other of the two signals branched in the optical combiner 211 is input to one of the N input ports of the second wavelength-selective switch 213, and wavelengths to be added are input through the input ports arbitrarily selected as add ports. And a wavelength-division multiplexed signal is output through one of the output ports. Note that in the case where wavelengths have been added through other input ports of the second wavelength-selective switch 213, those wavelengths of the branched signal from the optical combiner 211 can be blocked so that those wavelengths are not output from the output port of the second wavelength-selective switch 213.
The monitor 214 is used to monitor theoptical power of each of the wavelengths of the optical signal output from the second wavelength-selective switch 213. The control circuit 215 equalizes the optical powers of the wavelengths of the optical signal output from the second wavelength-selective switch 213, by controlling the variable attenuator function of the wavelength-selective switch 213, based on theoptical power of each of the wavelengths monitored by the monitor 214.
The WDM transmitter 220 shown in FIG. 11 includes a first wavelength-selective switch 221, a second wavelength-selective switch 222, an optical combiner 223, a monitor 224, and a control circuit 225. Unlike the case of the WDM transmitter 210 shown in FIG. 10, the optical combiner 222 is disposed after the wavelength-selective switches 221, 222.
That is, the first wavelength-selective switch 221, as with the wavelength-selective switches 201, 212 in FIGS. 9 and 10, includes a single input port and N output ports. Among the wavelengths of a wavelength-division multiplexed signal input to the input port, wavelengths to be passed through are output to the optical combiner 223 through the single output port, while wavelengths to be dropped are output through some of the output ports selected as drop ports. The second wavelength-selective switch 222, as with the wavelength-selective switches 202, 213 in FIGS. 9 and 10, has N input ports and a single output port. Wavelengths to be added are input through some of the input ports selected as add ports and are output to the optical combiner 223 through the single output port.
In the optical combiner 223, the wavelengths from the first wavelength-selective switch 221 and the wavelengths from second wavelength-selective switch 222 can be combined and output. The monitor 224 monitors the optical power of each of the wavelengths of the optical signal output from the optical combiner 223. The control circuit 225 equalizes the optical powers of the wavelengths of the optical signal output from the optical combiner 223, by controlling the variable attenuator function of each of the wavelength-selective switches 221 and 222, based on the optical power of each of the wavelengths monitored by the monitor 224.
Like the WDM transmitters 200, 210, and 220 shown in FIGS. 9 to 11, by employing dynamic optical add-drop multiplexers in which drop and add ports for a wavelength-division multiplexed signal can be arbitrarily selected independently of wavelengths, a WDM transmission system capable of flexibly setting transmission lines in wavelength units can be constructed.
On the other hand, optical transmission lines forming part of the aforementioned WDM transmission system become longer with development of communication networks, so that WDM transmitters are being requested to be constructed so that long-distance transmission can be performed. What matters is that in performing long-distance transmission of a wavelength-division multiplexed signal, losses in optical fibers and amplifier's gain vary from wavelength to wavelength. Such a difference in transmission loss or amplifier's gain between wavelengths can be the cause of a difference in light intensity between wavelengths at a receiving side.
The WDM transmitters 200, 210, and 220 shown in FIGS. 9 to 11 are attempting to compensate for such a difference in intensity between wavelengths by employing the variable attenuator function equipped, along with the port switching function, in the aforementioned wavelength-selective switch.
For instance, as shown in FIG. 12, in an optical transmission system 230, in which in-line amplifiers 234, 235 are disposed in a transmission fiber 233 between two nodes 231, 232 having the construction of the WDM transmitter 210 shown in FIG. 10, a wavelength-division multiplexed signal output from the node 231 (see “A” in FIG. 12) varies in light intensity from wavelength to wavelength due to the amplification characteristics of the in-line amplifiers 234, 235 (see “B” and “C” in FIG. 12), but the light intensities at different wavelengths are equalized by attenuating the wavelengths of the output optical signal, using the variable attenuator function of a wavelength-selective switch 213 provided in the node 232 (see “D” in FIG. 12).
However, as transmission distance is increased, a difference in intensity between wavelengths becomes greater due to the influence of transmission lines between WDM transmitters. As a result, there are cases where a great difference in intensity exceeding a dynamic range which can be corrected by the variable attenuator function of the aforementioned wavelength-selective switch must be corrected.
The WDM transmitters shown in FIGS. 9 to 11 are attempting to compensate for a difference in intensity by using the variable attenuator function of the aforementioned wavelength-selective switch. Therefore, in the case where a great difference in intensity exceeding a dynamic range which can compensate for a difference in intensity occurs within nodes, a compensation for this difference cannot be made sufficiently. Consequently, making transmission distance longer becomes fairly difficult.
For instance, as shown in “E” of FIG. 13, when an input optical signal has a great difference in intensity, there are cases where there is a range A which cannot compensate for a difference in intensity by the variable attenuator function of the wavelength-selective switch 213, as shown in “F” of FIG. 13.